1. Field of the Invention
The present invention relates to a Method, Process or System for more particularly polishing a radwaste feedstream or aqueous waste volume.
2. Background Information
The liquid radwaste (LRW) systems in commercial nuclear power plants and utilities have used conventional demineralization to provide discharge wastewater that attempts to meet both NRC and waste quality requirements for radioactive isotopes and RCRA elements or of a quality for recycle and reuse. The NRC (U.S. NUCLEAR REGULATORY COMMISSION) has particular interest in radioactive isotopes including Tritium and Strontium-90; and the RCRA (U.S. “Resource Conservation and Recovery Act”) elements include Ag (metal), As (metalloid), Ba (metal), Cd (metal), Cr (metal), Hg (metal), Pb (metal), and Se (nonmetal). It is known in the technology that radioactive fluids such as nuclear reactor cooling fluids and others characteristically have, along with Boron, salts and nonradioactive constituents. As indicated below, it is an object of the present invention “to maximize boron control”, accomplished in most instances within the scope of the invention by passage or reuse through the RO system; to meet discharge, recycle or reuse purposes. It is well understood in the technology that Boron is a neutron absorption agent which is consumed in controlling the nuclear reaction at a nuclear plant facility. Therefore, boron control and passage in the present invention is important to the nuclear reaction itself. Importantly, Boron passage is maximized utilizing the present invention while past technology decreased the passage of Boron. Salts and nonradioactive metals are passed on through the present invention as much as possible to reduce the volume of radwaste generated. In this regard the present invention is utilized as the sole release point for liquid effluents from a nuclear power plant. Liquids must be released as a license condition for the operation of the plant. B10 (B10) is consumed as a neutron absorption agent to control the nuclear reaction and is converted to B11 (B11). While prior technology teach retaining B11 the present invention's method functions to remove the B11 and assist revitalization of the neutron absorption capacity of the reactor coolant with the addition of fresh B10. This is an important distinction of the present invention in relation to past technology. Plants have attempted to stay below regulatory discharge requirements by using demineralizer (ion exchange) technology where the principal components and technology have included conventionally positioned demineralizing systems consisting of tanks, essential piping, pumps and valves. Liquid wastes generated in using such past conventional setups have required further cleanup or treatment before release or recycle. They have been collected and processed by either installed plant demineralizers or vendor-supplied, conventional demineralization systems which were not capable of effectively treating or processing radwaste liqiuids or fluids to quality grade water permitting discharge back to the outside environment.
More recently other factors beyond regulatory requirements have come into play. The potential contamination of rivers, lakes, bays and estuaries by plant discharges has resulted in increased nuclear insurance rates, and attention by industry peer review and audit groups; and public attention by various activist groups has motivated utilities to continually improve environmental performance, by reducing releases of the main or primary gamma activity contributors of cesium, cobalt and iodine far below regulatory level for liquid releases to the environment.
As discharge of gamma emitters was reduced, secondary isotopes such as tritium and iron, that were previously masked or difficult to detect, became highlighted. Iron isotopes 55 and 59 were not readily removed by filtration and ion exchange. Therefore, a need for a more effective removal processing before discharge became necessary.
Tritium isotopes were more readily detected but virtually impossible to separate from the process stream. Therefore, plants have been evaluating the possibility of recycle of liquid streams back to the plant for further use instead of being discharged with tritium to the environment.
Prior to this invention, mechanical filtration and ion exchange technology was marginally capable of meeting reduced gamma discharges, effective removal of secondary isotopes for discharge or production of the high quality water required for recycle to allow retention of tritium.
Also, prior to the present invention, thousands of pounds of Boron were being discharged each year to the environment.
Additionally, aspects of the prior art in dealing with RCRA (“Resource Conservation and Recovery Act”) wastewaters and recycle applications containing regulated chemicals permitted only to be discharged in limited amounts, have been sorely wanting.
In recent decades various membrane separation processes have been developed and utilized in the field of potable water purification; and more recently in the treatment of various process and waste liquors. Some of the membrane processes are capable of removing both dissolved and particulate contaminants. The best known and most utilized membrane processes in the field of water and wastewater treatment are those utilizing pressure gradient as the process driving force. These processes include reverse osmosis, nanofitration, ultrafiltration and microfiltration.
Pressure driven membrane separation processes differ mainly in the pore size of their membranes, which makes a particular membrane effective for the removal of a specific range of impurities. Reverse osmosis is used to remove all ionic species and thus has the smallest membrane pore size.
Direct Osmosis is the spontaneous transport (diffusion) of solvent that takes place when two solutions of different concentrations are separated by a semipermeable membrane that allows the solvent (but not dissolved species) to pass through it. The solvent, usually water, flows through the membrane from the less concentrated into the more concentrated solution. The flow continues until the solutions on both sides of the membrane are at the same concentration or until the pressure exerted by the difference in height between the two solutions is sufficient to stop the flow. The pressure required to just stop the flow is termed osmotic pressure, at which point the two solutions are in equilibrium. As the concentration difference between the solutions on the two sides of the membrane increases, the osmotic pressure increases.
This direct osmosis process can be reversed. Pressure applied to the more concentrated solution will permit the solvent to flow through the semipermeable membrane into the less concentrated solution. The term reverse osmosis (RO) has substantially been reserved, under conventional standards, for separation of dissolved ions and small molecules that contaminate aqueous solutions. The pressure exerted to force the flow of water into the less concentrated solution must exceed the osmotic pressure of the feed solution.
In reverse osmosis (RO), pressure greater than the osmotic pressure (usually about 10-40 MPa) is applied to the concentrated solution to cause the solvent to flow from the concentrated side of a semipermeable membrane to the diluted side. In most applications of RO, when the dissolved solid concentration reaches about 1-5 wt %, the osmotic pressure becomes too high to sustain the process. RO has been found to typically remove 90-99.5% of total dissolved inorganic solids and 70-99% of dissolved organic solids.
In the Prior Art, RO is considered a well-developed technology with systems having been used in the past in many industrial settings for years; having been applied to separation, concentration of product streams and wastewater treatment. More specifically, the technology has been used for removal of radionuclides from low level liquid wastes such as waste streams at nuclear power plants. In recent years RO systems have been used to replace or augment existing ion exchange technology. RO systems in the nuclear industry are usually a part of an overall liquid waste treatment system.
Therefore, over many years, application of pressure driven membrane separation RO has been demonstrated in nuclear and non-nuclear wastewater applications. In such uses, RO was placed in a position in a system or process ahead of a polishing demineralization treatment, in a configuration referred to as RO-IX. In this configuration, the RO separated the raw process stream into two distinct streams: 1) the permeate (clean) and 2) the reject (dirty) stream. The clean stream, containing reduced concentrations of impurities after one or two passes, still often required polishing with downstream ion exchange media to meet quality requirements for either recycle or discharge. The reject stream, containing a high fraction of the raw stream constituents, was then subject to further processing, for example, drying to dried solids. In this configuration, 90% to 99% of the waste stream contaminants were rejected by the RO and the balance removed by the downstream IX polisher.
A number of deficiencies of this process configuration have been identified. When presented with a high concentration of contaminants in the raw waste stream, a higher than desirable concentration of contaminants pass through the membrane, often requiring multiple RO passes and extensive ion exchange polishing to produce an acceptable permeate, thus resulting in increased capital equipment and operating costs and increased consumption of ion exchange resin to polish the permeate. In some instances, the RO concentrated raw stream contaminants to near their solubility limits. The volume of the reject had to be increased to preclude precipitation on the membrane and their fouling. The need for these process conditions, to effect adequate cleanup, resulted in increased equipment and operating costs, larger equipment footprint and increased resin consumption and further processing of increased reject volumes.
A problem, however, was found to occur with respect to utilization of the reject stream, which generally represented 2-15% of the feed stream volume. It was found that it was required to deal with such a waste stream because it contained a number of undesirable elements. In non-nuclear applications these elements could often be discharged to sanitary sewers or even returned to the original body of water. However, in nuclear applications the reject contained radioactive isotopes that were required to be sent to special disposal facilities in a stabile form. Since water, or substantially aqueous volumes were not regarded as an acceptable form, the water had to either be solidified or evaporated to leave dry or dirt-like solids. Since these volumes were significant the cost was found to be high, and sometimes prohibitive.
Likewise, the reject stream suffered shortcomings from this processing configuration. The reject stream, generally representing 2-15% of the feed stream volume, contained 90% to 99% of the contaminants presented to the RO in the raw waste stream. Some constituents may have been near their solubility limits, and thus could not be reprocessed through the RO a second time. If the reject was returned to the plant, the high concentration of contaminants was undesirable. In the past, in the instance of radioactive waste, if the reject was further processed for disposal, the high concentration of contaminants might cause elevated radiation doses. In other instances, the high concentration of specific isotopes might cause an increase in waste classification from the desirable low Class A classification to Class B and C, resulting in more onerous packaging, shipping and disposal requirements. In some past instances, the concentration of certain radioactive constituents caused the classification to be greater than Class C (GTCC), which precluded disposal by any means.
Additionally, in instances where the reject volume was increased to preclude exceeding the solubility of raw waste stream constituents to prevent membrane fouling, the greater volume of reject resulted in increased secondary processing costs for such treatment processes as evaporation, drying or solidification. The increased reject volumes and secondary processing costs reduced the economic viability of the RO-IX process configuration.
In non-nuclear applications these concentrated reject constituents precluded normal discharge to sanitary sewers, or return to the original body of water.
If these existing permeate and reject stream problems could be solved, substantial operational and economical benefits would be realized, thus enhancing the value and utility of RO as a purifying method, including RO treatment of nuclear waste or process streams. Therefore, the teachings of the present invention were developed to overcome the problematic issues in the prior art regarding these aspects.
Accordingly, it is an object of the present invention to provide a permeate stream and a reject stream, so that the reject is recycled back to the front of a system of which it is a part of, or back to the plant source, so that the removal percentage of demineralizers employed in such a system is great enough to prevent excess buildup of most all isotopes and other fouling chemicals.
It is also an object of the invention, in preferred embodiments, to utilize a method and system in proximate location to the end of a demineralizer radwaste system to polish wastewater of the remaining isotopes and other dissolved and colloidal materials, when present.
It is a further object of the invention to maximize boron control. In most instances this is accomplished, within the scope of the invention, by passage or reuse through the RO system. This is done to meet discharge, recycle or reuse purposes. This is accomplished by raising or lowering the pH for the purpose of altering the characteristics of boric acid passage.
A further object exists in facilitating the periodic use of a chemical treatment system to precipitate the silica in the system, or capture the silica by selective ion exchange when required.
Further objects and advantages include utilizing the RWRO post IX polisher therein of the invention; the advantageous use of carbon filters and ion exchange resins as good removal media for TOC found in the feed water; the reduction of TOC fouling of RO membranes; the use of deep bed carbon filtration ahead of demineralizers to reduce fouling as compared to mechanical filters conventionally used ahead of RO systems; the reduction of TSS fouling of RO membranes; the reduction of dose rate on RO system and membranes; the reduction of activity returned to the plant or discharged to the environment; the maximization of resin utilization by exposing the resin to higher influent activity concentration and retaining resin until completely depleted; and the utilization of acid conditions by cation resin (i.e., hydrogen form cation resin before the RWRO) to reduce pH adjustment and improve BA passage.
Among the many advantages to RO performance and to rejecting back to a plant or facility with which it is associated; include, but are not limited to: the reduction of volume rejected and returned to the plant; the reduction in the waste classification of concentrates and/or resulting dried solids; the maximization of resin utilization by exposing resin to a higher influent activity concentration; the aspect of permitting ionic through prior to resin replacement; the prevention of the return to the plant, or environmental discharge, of difficult to remove isotopes; the facilitation of scavenging of targeted isotopes, such as antimony, in a smaller waste stream, then provided; and the decrease on osmotic load through the use of increased permeate flow.
It is a further object of the invention to more effectively and efficiently utilize, in preferred embodiments, mechanical filtration and ion exchange technology in meeting reduced gamma discharges, removal of secondary isotopes for discharge and production of high quality water required for recycle to allow retention of tritium.
It is a further object to provide effective boron recycle. A related object, in preferred embodiments pertains to capture, for reduction of discharges or beneficial recycle, of non-radioactive elements such as boron.
A further object exists in providing a system which has application in dealing with RCRA wastewaters and recycle of regulated chemicals that can be discharged only in limited quantities.
It is the object of the present invention to provide a methodology of applying RO and ion exchange (IX) in such a manner as to create more favorable operating conditions and performance of the RO and demineralizer (IX) by situating the demineralizer system ahead of the RO in a configuration referred to as IX-RO wherein the RO acts as a polisher to the demineralization system. In the IX-RO configuration, often 90% to 99% of the contaminants will be removed by the IX system in advance of the RO polisher. A secondary configuration provides demineralization of the reject stream to further strip undesirable species from the reject before recycling or processing of the reject stream. These configurations result in improved RO performance and improved permeate and reject stream quality and conditions.
Another object and advantage of the invention includes IX scavenging of raw waste stream constituents that, when concentrated in the RO, could precipitate and foul the RO membranes. Reduction of such agents permits controlling the reject flow rates to meet the hydraulic needs of the membranes instead of adjustment to control internal RO precipitation. This results in reduced reject flow rates and volumes that must be recycled or treated.
A further object and advantage is served by IX scavenging of raw waste stream constituents, resulting in reduced concentration of contaminants in the reject stream. This favorable condition reduces the inventory of contaminants recycled back to the plant or the process system itself. The lower concentration of reject contaminants permits reprocessing through the IX-RO system with reduced concern of precipitation.
Another object and advantage of the invention is set forth by IX scavenging of raw waste stream constituents, including reduction in the concentration of key isotopes that affect waste classification for waste packaging, shipping and burial. Reduction of these isotopes in the raw waste stream, results in sympathetic reduction in the reject stream subject to secondary processing for disposal.
A further object and advantage of the invention is served by IX scavenging of raw waste stream constituents, including radioactive isotopes, to reduce mechanical entrapment of isotopes in the RO membranes and equipment system.
Accordingly, the lower radioactive dose rates achieved reduce exposure to operating personnel. This also reduces the frequency of, or need for, cleaning RO membranes, which would normally be needed to address and lower accumulated substances causing radioactivity. This facilitates avoidance of exposure to personnel, which would otherwise be necessary for handling and packaging of expended membranes.
Further objects and advantages of the invention are served by IX scavenging of contaminants from the reject stream to reduced isotopic content, resulting in reduced dose buildup during secondary process of the reject stream. Particular isotopes; for example, cesium 137, cobalt 60, carbon 14, iron 55, and antimony 125, when present in elevated concentrations, may result in elevated waste classification imposed by the Nuclear Regulatory Commission and other governmental and regulatory bodies. This results in more onerous packaging, shipping and burial requirements. Scavenging, by IX and similar processes, for these and other select isotopes from the reject stream will enable maintenance of Class A classification, resulting in reduced handling and disposal costs.
Additional objects and advantages of the invention include IX scavenging of raw waste stream constituents, to reduce the concentration of contaminants presented to the RO. Lower RO influent contaminates results in reduced contaminant concentrations in the RO permeate. This reduces, or eliminates the need for additional RO passes to effect cleanup of the process stream; or, in turn, reduces or eliminates the need for post-RO IX polishing of the permeate for discharge or recycle.
Other objects and advantages of the invention are served by IX scavenging of raw waste stream constituents to reduce the concentration of contaminants presented to the RO. This results in contaminants being affixed or captured by the IX system. This is a more favorable and efficient waste form for handling, packaging and disposal.
An additional object of the invention, and advantage served, includes IX scavenging of raw waste stream constituents, where the RO acts as a polisher. This maximizes IX resin utilization by permitting IX beds to remain in service well beyond normal chemical depletion. Ionic leakage from partially or fully chemically depleted IX bed is either captured by downstream IX bed; or when passed to the RO, is rejected to the reject stream. This contribution to the reject stream, in this case, is sufficiently small, often 1%-2% of the influent stream contaminate total. This does not materially or adversely affect the quality and secondary processing of the reject stream.
A further object, and advantage, of the invention lies in maximizing resin utilization and consumption reduction wherein the IX beds are exposed to the maximum concentration of contaminants of the raw process stream. This results in a more complete utilization of the exchange capacity of the resin and a higher exchange equilibrium. All of these things result in higher waste loading and reduced resin consumption, compared to IX performance when positioned in the dilute contaminant stream of the RO permeate as was the case in the Prior Art.
Another object of the invention is to set forth IX positioning in advance of the RO to enhance boron passage through the RO membranes. This enhanced passage is affected by pH reduction as the process stream passes through hydrogen form cation resin or by addition of acids, such as sulfuric or hydrochloric, to the process stream. Passage of a greater fraction of the boron to the permeate permits recovery of boron in the permeate and reduces the concentration in the reject stream. This avoids RO operating limitations imposed by the potential for precipitation of boron; and also reduces the resulting dried waste volumes due to reduced boron content. In the alternative, when maximizing the concentration of BA in the reject stream or minimizing boric acid in the permeate; the pH can be increased by passing the process stream through a hydroxide form anion resin or by addition of a basic solution such as sodium hydroxide.
A further object includes setting forth a system where the IX position is placed in advance of the RO, in facilitating the periodic use of a chemical treatment system to precipitate silica in the IX system, or for capturing the silica by selective ion exchange when required or appropriate.
An additional object is to set forth a method and system where the IX position is placed in advance of the RO in facilitating improved TOC removal from carbon filters downstream. When employed such carbon filters have the primary purpose in filtration and TOC removal. The use of IX aids in removal of that fraction of TOC that successfully transits the carbon filter. This protects the downstream RO from TOC contamination (a primary membrane fouling mechanism).
Yet a further object of the invention includes presenting a system and process that provides the further advantage of utilizing the RWRO as a post IX polisher in the invention. Other related advantages, in this regard, include the advantageous use of carbon filters and ion exchange resins as removal media for TOC found in the feed water; the reduction of TOC fouling of RO membranes; the use of deep bed carbon filtration ahead of demineralizers to reduce fouling as compared to mechanical filters conventionally used ahead of RO systems; the reduction of TSS fouling of RO membranes; the reduction of dose rate on RO system and membranes; the reduction of activity returned to the plant or discharged to the environment; the maximization of resin utilization by exposing the resin to higher influent activity concentration and retaining resin until completely depleted; and the utilization of acid conditions when needed.
It will, therefore, be understood by those skilled in these technologies that substantial and distinguishable device, process and functional advantages are realized in the present invention over the prior art. It will also be appreciated that the efficiency, flexibility, adaptability of operation, diverse utility, and distinguishable functional applications of the present invention all serve as important bases for novelty of the invention, in the field of improved aqueous waste and radwaste treatment.